1. Field of the Invention
The invention relates to an image edge enhancement method applied to a grid matrix output device, and in particular to a color image edge enhancement method, which is used to determine how to proceed with the dotting of the edge pixels of the color images, so that the edges of the output images can be made smoother.
2. Related Art
Recently, in the field of image processing, the key point of most research and development is to improve the visual effect of an image output apparatus (e.g. a printer) such that the visual effect generated after image output matches with the real subject. The edge of the original image object, e.g., texts, curves, three-dimensional objects, etc., can be approximated by the linear equation or natural logarithm, etc., to be more smooth. When these objects are digitally processed, they are required to be converted into a matrix form which is processable for an image output apparatus, and their edges are required to match the grids of the apparatus.
However, if the resolution of the image output apparatus is relatively low, the edge of the image object is found to be step-shaped of one grid by another grid through the naked eye, and a smooth edge as that of the original image object cannot be obtained. Therefore, various solutions are proposed to reduce the jagged effect on the edge of the image object, such that the image output by the image output apparatus can match with the real subject.
As for Edge Enhancement Method and Apparatus for Dot Matrix Devices disclosed in U.S. Pat. No. 5,029,108, the dotting state of the pixels of an image edge and that of the surrounding pixels are compared with a plurality of predetermined pictures, to determine which predetermined picture the pixels of the image edge match with, thus it is determined how to dot the pixels at the image edge to smooth the output image edge and to allow the image more accurately match with the original one. The enhancement of central pixels of each predetermined picture will be modified differently according to the dotting state of the surrounded pixels, for example, dotting at ¼ to the left of the central pixel, dotting at ⅔ to the right of the central pixels, etc
However, the method mentioned above is only suitable for single color image output (namely, the black and white image output). When executing the color image output of the image output device, since each of the color images are composed of the planes of four different colors (cyan, magenta, yellow, and black; CMYK), Thus, in executing the supplementary dotting of the pixels on the edge of the image, the planes of all the four colors must be taken into consideration. Presently it doesn't have any mechanism, which can be utilized to determine the planes, of which colors must be utilized to implement the supplementary dotting of the pixels on the edge of the image. However, if the planes of all the four colors are used to perform the supplementary dotting, then the color displayed on this pixel will be black, thus it may be different from that of the original subject. Nevertheless, there is just no way to know which color may be used to implement the supplementary dotting.
Therefore, to the color image output device, the determination of how to enhance the edge of the image, so that the edge of the output image is smoother and with its color matching that of the original subject, is the most important problem that must be solved in this field.